This invention relates to pulse oximetry devices and, more particularly, to an improved fetal pulse oximetry probe and oximeter.
Pulse oximeters are typically used to measure various blood characteristics including arterial blood oxygen saturation and pulse rate. Pulse oximetry devices typically comprise a non-invasive probe which passes light through a portion of the patient's tissue where blood perfuses the tissue, and which photoelectrically senses the absorption of light in the tissue. The detected light is then used to determine the characteristic of interest.
In plethysmography (e.g., pulse rate and amplitude) and pulse oximetry (blood oxygen saturation), the light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood. The amount of light passed through the tissue varies in accordance with the amount of blood and blood constituents in the tissue. For measuring blood oxygen saturation, such sensors are provided with light sources at two or more wavelengths and a photodetector that is adapted to operate at those wavelengths in accordance with known techniques for measuring blood oxygen saturation. See, e.g., U.S. Pat. No. 4,653,498 issued to New, Jr., et al. incorporated herein by reference.
Pulse oximetry probes generally fall into two categories. Transmissive pulse oximetry probes shine light through opposed blood perfused tissue surfaces, such as a finger or ear, emitting and detecting light on opposite sides of the tissue. Transflectance probes both emit light into and detect light from the same side of the blood perfused tissue.
Pulse oximeters may be used to measure fetal blood oxygen saturation in utero during labor and delivery. Since the presenting part of a fetus (usually the head) does not offer opposed tissue surfaces for transmissive pulse oximetry, transflectance probes are often used. However, transflectance pulse oximetry probes are quite susceptible to inaccuracies caused by shunting of light between the emitter and detector, i.e., light which travels from the probe's emitter to the detector but which bypasses the blood perfused tissue. Shunting of light may occur for many reasons.
FIG. 1 shows a transflectance pulse oximetry probe 10 comprising a light emitter 14 and light detector 18 disposed within a housing 22. Probe 10 is disposed in close proximity to a body 26 which includes blood perfused tissue layer 30, bloodless tissue layer 32, and skin 34. Path 1 shows light which fully penetrates the blood perfused layer 30. Now assume that probe 10 is in poor contact with skin 34, and that there is extraneous matter 38 such as hair, mucus, etc., disposed between probe 10 and skin 34. When the probe is in poor contact with the skin, light may be directly reflected from the top surface of the skin or piped through the extraneous matter as shown by path 2. Alternatively, the light may scatter below the surface of skin 34 but may not travel deep enough to penetrate blood perfused layer 30. Instead, the light travels through layer 32 as shown by path 3. The depth uniformity and/or location of layer 32 is typically affected by local vasoconstriction, excessive force applied to the back surface of the probe (which locally exsanguinates blood from the tissue beneath the probe), site-to-site variations of the distance to the blood perfused layer or the lack of blood vessels in the region. Thus, unless the placement of the surface probe is well controlled in an environment free of excessive applied force and other light shunting causes, accuracy of the calculated saturation will be suspect.
Pulse oximetry in the fetal environment is challenged by the above factors, and is further complicated by the fact that the intrauterine sensor site is remote and cannot be directly observed. Consequently, knowledge about the quality of the site and the sensor to tissue contact is either difficult or simply unavailable, and the oximetry data is sometimes unreliable.